Microfluidic systems offer potential advantage in applications such as chemical synthesis, distillation, and analysis. Numerous microfluidic systems have been developed, wherein minute volumes of fluids are mixed, separated, heated, reacted, and the like. In order to control such processes, it is imperative that each chemical component be controlled. This requires, fundamentally, an ability to detect the presence of a chemical in a fluid. Further, it is advantageous to be able to monitor the chemical concentration in realtime as well.
In general, a chemical component can not, itself, be directly measured. As a result, detection methods rely on the detection of a change in a measurable property of the chemical, such as electrical conductivity, viscosity, color, absorption, refractive index, and the like. As such, chemical components are often referred to as analytes. An analyte is a substance or chemical constituent that is determined in an analytical procedure, such as a titration. For instance, in an immunoassay, the analyte may be the ligand or the binder, while in blood glucose testing, the analyte is glucose. For the purposes of this specification, including appended claims, the term analyte is used to refer to a chemical or biological component present in a carrier solution, such as a buffer solution.
There are many known methods for detecting the presence of an analyte in a fluid. These include fluorescence microscopy, visual inspection of the fluid, monitoring electrical conductivity of the fluid, and optical detection of a change in an optical property of the fluid. In most prior-art systems, however, these methods rely on the detection of a very small change in a property of the solution. To complicate matters further, these methods often provide an output signal that varies in linear fashion, commensurate with the small change in the property. As a result, the signal strength associated with the presence of the signal is often low and thus the signal-to-noise ratio of the output signal is poor. This limits the sensitivity of such detection methods.
Fluorescence microscopy offers high sensitivity detection of an analyte; however, it requires the existence of a suitable fluorescent molecule that can be attached to the analyte. In a typical fluorescence application, an analyte is selectively labeled with a fluorescent molecule called a fluorophore (e.g., green fluorescent protein (GFP), fluorescein or DyLight 488, etc.). The “tagged” specimen is illuminated with light of a specific wavelength (or wavelengths) which is absorbed by the fluorophores. The absorbed light causes them to emit light at a different color than the absorbed light. The presence of the analyte is indicated when this emitted light is detected. A fluorescence detection system normally requires a microscope, light source (e.g., a xenon arc lamp or mercury-vapor lamp), multiple color filters, and a dichroic mirror or beamsplitter (i.e., a mirror that reflects one color but not another). The filters and the dichroic elements are chosen to match the excitation and emission wavelengths of the fluorophore used to label the specimen. Although capable of high sensitivity, a fluorescence detection system is quite complicated and can be very expensive.
Other optical detection methods generally rely upon passing a light beam through the fluid and detecting a change in absorption or refractive index through detecting a change in the intensity of the light beam. There are several drawbacks to such approaches, however. First, the change in the measured optical property is typically small. In order to obtain a detectable output signal, therefore, a large amount of fluid must be interrogated. As a result, the detection region of such systems is typically large. As an analyte begins to reach the detection region, its concentration is non-uniform and initially very low. Since only a small portion of the detection region contains analyte, its effect on the entire light beam is muted. This reduces the effective chance induced by the arrival of the analyte. As the analyte continues flowing into, and fills, the detection region, the concentration level slowly builds and the output signal slowly changes until it is detectable. These factors lead to an uncertainty into the time at which the analyte has arrived. In addition, the need to interrogate a large amount of the fluid increases the cost of such a system since its overall size must be increased to accommodate the larger detection region.
Secondly, since these methods require that optical energy pass through the solution itself, they are inappropriate for use in some applications. For example, some analytes are sensitive to light and may be damaged or chemically altered when exposed to such energy.
There exists a need, therefore, for chemical detection that is highly sensitive and that can rapidly provide a discernable output signal.